This invention relates generally to the confinement of molten metal and is particularly directed to the casting of metal sheets using an electromagnetic field to form the casting mold.
Steel making occupies a central economic role and represents a significant fraction of the energy consumption of many industrialized nations. The bulk of steel making operations involves the production of steel plate and sheet. Present steel mill practice typically produces thin steel sheets by pouring liquid steel into a mold, whereupon the liquid steel solidifies upon contact with the cold mold surface. The solidified steel leaves the mold either as an ingot or as a continuous slab after it is cooled typically by water circulating within the mold wall during a solidification process. In either case, the solid steel is relatively thick, e.g., 6 inches or greater, and must be subsequently processed to reduce the thickness to the desired value and to improve metallurgical properties. The mold-formed steel is usually characterized by a surface roughened by defects, such as cold folds, liquation, hot tears and the like which result primarily from contact between the mold and the solidifying metallic shell. In addition, the steel ingot or sheet thus cast also frequently exhibits considerable alloy segregation in its surface zone due to the initial cooling of the metal surface from the direct application of a coolant. Subsequent fabrication steps, such as rolling, extruding, forging and the like, usually require the scalping of the ingot or sheet prior to working to remove both the surface defects as well as the alloy deficient zone adjacent to its surface. These additional steps, of course, increase the complexity and expense of steel production.
Steel sheet thickness reduction is accomplished by a rolling mill which is very capital intensive and consumes large amounts of energy. The rolling process therefore contributes substantially to the cost of the steel sheet. In a typical installation, a 10 inch thick steel slab must be manipulated by at least ten rolling machines to reduce its thickness. The rolling mill may extend as much as one-half mile and cost as much as $500 million.
Another approach to forming thin metal sheets involves casting into approximately the final desired shape. Compared to current practice, a large reduction in steel sheet total cost and in the energy required for its production could be achieved if the sheets could be cast in near net shape, i.e. in shape and size closely approximating the final desired product. This would reduce the rolling mill operation and would result in a large savings in energy.
There are several technologies currently under development which attempt to achieve these advantages by using an electromagnetic field to form the steel sheet in the casting process. Some of the approaches under investigation use electromagnetic energy entirely, and others use electromagnetic energy in conjunction with a solid mold on one or both sides of the sheet. For example, the use of electromagnetic levitation techniques has been employed in the aluminum industry. The practice there is to use electromagnetic fields to contain the top inch or so of a large, thick ingot. The molten aluminum is cooled and solidified before it touches any mechanical support. Examples of this approach can be found in U.S. Pat. Nos. 3,467,166 to Getselev, 4,161,206 to Yarwood et al., and 4,375,234 to Pryor. Other examples are U.S. Pat. Nos. 4,678,024 to Hull et al. and 4,741,382 to Hull et al. The Hull patents describe use of alternating electromagnetic fields to levitate an entire sheet of molten metal for horizontal casting.
There are several difficulties associated with the use of electromagnetic fields as a substitute for solid wall molds. Such difficulties include high energy requirements, large eddy currents, instabilities, and shaping the electromagnetic field to conform to the desired shape of the mold. For example, the Getselev patent describes a device for electromagnetic confinement of a metal, in particular aluminum, as it is cast into rods. The Getselev device employs metallic rings which form screens located at specific positions around the molten metal. These screens serve to shape and modify the magnetic field. A frequency of the alternating field in Getselev is chosen to make the skin depth about 1/3 of the horizontal distance to the center. Eddy currents are generated in the molten aluminum to interact with the applied field and produce a containing force at the surface. In addition to these desirable eddy currents in the aluminum, the Getselev device also sets up currents in the ring and screen. These currents are responsible for shaping the field but result in large power losses. In addition, the large magnetic fields in the air near the caster may interfere with other equipment and may be a safety hazard.
Another example of an electromagnetic casting process is U.S. Pat. No. 4,414,285, "Continuous Metal Casting Method, Apparatus and Product", by H. R. Lowry et al., Nov. 8, 1983. Lowry et al. describes a continuous casting process which a single magnet with AC conducting coils carries three-phase, high frequency current and produces a magnetic field which partially levitates and confines a molten metal cylinder. The Lowry device applies the levitating force only at the surface of the cylinder. Moreover, the process of Lowry is not moldless. The confining force reduces but does not eliminate the pressure of the molten metal on the mold.
Another of the previous methods is described in the aforementioned patents by Hull, et al. The Hull patents describe how molten steel could be poured through and solidified in an electromagnetic caster in a horizontal geometry. A horizontal geometry has the advantage of low eddy currents but the stability of the molten metal in the field can be weak.
The stability problems of horizontal casting can be overcome by vertical casting. With vertical casting, a metal could be cast in a vertical position with a high frequency spatially varying magnetic field equalizing the ferrostatic pressure head at each vertical position. A disadvantage of the vertical casting method is that when applied to thin sheets, very high frequencies are required and a large amount of heating is generated.
Accordingly, an objective of the present invention is to provide a magnetic field which can retain a molten metal with smooth, even vertical boundary suitable for casting.
A further objective of this invention is to electromagnetically cast steel sheet with a minimum of electromagnetic heating of the molten and solid steel and to provide a casting system with the molten metal in stable mechanical equilibrium within the caster.
A further objective of this invention is to produce an electromagnetic levitation method that combines the low heat production of horizontal casting with the strong stability of vertical casting.
Another object of this invention is to produce steel sheet that requires little or no rolling after the casting operation.
Still another object is to produce steel that has good metallurgical properties and a good surface quality directly upon leaving the caster.
A still further object is to cast molten steel in such a manner that the surface skin solidifies without mechanical contact with a mold or roller.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.